The present invention relates generally to an ultra-sonic sensor to be employed in an automotive suspension control system. More specifically, the invention relates to an ultra-sonic sensor system suitable for detecting rolling and/or pitching motion of the vehicle and controlling the automotive suspension in accordance with the intensity of rolling and/or pitching.
In the recent years, various automotive suspension control systems attempting to improve riding comfort without affecting driving stability have been proposed and developed. Some of such suspension control systems control damping characteristics of the suspension system depending upon the magnitude of rolling and/or pitching motion of the vehicle.
Conventionally, such rolling and/or pitching motion of the vehicle have been monitored indirectly using various vehicular driving parameters other than the vehicle body behaviour per se. For instance, in order to detect rolling motion of the vehicle, lateral force, steering angular displacement and so forth are observed. On the other hand, in order to detect vehicular pitching motion, application of the brakes, acceleration and deceleration of the vehicle and so forth are observed.
On the other hand, the co-pending U.S. patent application Ser. No. 647,648, filed on Sept. 6, 1984 now abandoned and assigned to the assignee of the present invention discloses an electronic suspension control system employing an ultra-sonic sensor for monitoring road surface conditions for use in road roughness dependent suspension control. In the disclosed system, ultra-sonic waves are transmitted toward the road surface and the ultra-sonic waves reflected by the road surface are received. In theory, the ultra-sonic sensor system measures the elapsed time between an ultra-sonic waves transmission and reflected ultra-sonic waves reception. Based on the measured elapsed time and the known propagation speed of the ultra-sonic waves, the distance can be arithmetically derived. In practice, the measurement of the elapsed time starts in response to the onset of transmission of the ultra-sonic waves. In order to avoid the influence of noise, an ultra-sonic waves receiver signal, the signal value of which is dependent upon the received intensity of ultra-sonic waves, is compared with a predetermined threshold value. When the receiver signal level exceeds the threshold level, the measurement of the elapsed time is terminated and the measured elapsed time value is latched. However, the received intensity of ultra-sonic wave tends to fluctuate depending upon the external condition, such as atmospheric temperature and so forth. This makes the result of measurement of the elapsed time inaccurate.